‘Living’ anionic polymerization is the most useful technique for the synthesis of block copolymers because of the absence of transfer and termination reactions. Among different block copolymers, ABA triblock copolymers is an important class e.g. the most well known being styrenic thermoplastic elastomers with two glassy end blocks connected to an amorphous polydiene block. One of the most versatile methods for the synthesis of such triblock copolymers is the use of a dicarbanionic initiator with a two-step sequential monomer addition sequence. However, one major difficulty met is the limited solubility of dicarbanionic initiator in non-polar solvent media that is required for the preparation of a polybutadiene or polyisoprene central block with a microstructure constituted of a high percentage of 1,4-polybutadiene or 1,4-cis-polyisoprene units, which is required for optimal elastomeric properties. The latter goal explains why a lot of patents and papers have claimed or attempted the synthesis of various carbanionic species and particularly non polar solvent-soluble organolithium initiators.
Development of an ideal difunctional organolithium initiator, usable in hydrocarbon solvents for the anionic polymerization of dienes and/or vinyl aromatic hydrocarbon monomers has been a continuing effort since the past four decades. Because of the strong association of organolithium compounds, most of the dilithium initiators require some amount of polar additives to make them soluble in hydrocarbon solvents. Adduct of 1,3-divinylbenzene with sec-BuLi was the first example of difunctional initiator studied (U.S. Pat. No. 3,862,251 (1974), C. R. Hebd. Seances Acad. Sci. 283, 123 (1976), Plaste Kaustch, 26, 263 (1979), Makromol. Chem., 1985, 186, 2017). A mixture of soluble mono- and dilithiated and oligomeric species, with functionality higher than two due to possible polymerization of divinylbenzene, was obtained which did not guarantee a good control of diene polymerization. Another dianionic initiator was a bis-adduct of BuLi onto m-diisopropenylbenzene, which was an efficient bifunctional initiator in apolar solvent having good control over the molar mass and narrow molar mass distribution even in the presence of mixture of multiadduct, diadduct and unreacted sec-BuLi (Makromol. Chem., 1978, 179, 551; Polymer, 1982, 23, 1953). To avoid precipitation of the initiator was added σ-complexing polar agent such as triethyl amine (Macromolecules, 1977, 10, 287; Polymer, 1979, 20, 1129). However, even in the presence of triethyl amine, a mixture of species was still observed after the reaction between m-diisopropenylbenzene and sec-BuLi. Some other σ-complexing polar agents such as diethyl ether, tert-butyl methyl ether, N,N,N′,N′-tetramethylethylenediamine (TMEDA) and THF were found to be efficient polar additives, but leading to a high 1,2-microstructure of the polybutadiene block (Macromolecules, 1997, 30, 4254). It was observed that combination of the initiator seeding technique and weakly polar additives such as tert-BuOLi and anisole was necessary to prevent the presence of residual initiator and achieve equal reactivity of both end active centers so as to obtain SBS triblock copolymers with high content of 1,4-polybutadiene units (Macromolecules, 1997, 30, 4254; Macromolecules, 1997, 30, 7356). π-Complexing agents such as 1,2,4,5-tetramethylbenzene (durene) or tetraphenylethylene (TPhE) were also proposed which do not interact so strongly as σ-complexing agents with Li+ cation to bring about the dissociation of the organolithium aggregates (Polymer, 2003, 44, 4109; Polymer, 2003, 44, 6205, Polymer, 2005, 46, 303). Other researchers concentrated their efforts on the reaction of double diphenylethylene-type molecules with a stoichiometric amount of sec-BuLi in non-polar solvents (Polym. Prepr., 1984, 25 (2) 85). It was demonstrated that this difunctional initiator was efficient in the case of butadiene polymerization, but only at low monomer conversion. Dilithium initiator based on 1,3-di(1-phenylethenyl)benzene (PEB), although soluble in hydrocarbon solvents, led to bimodal molar mass distribution for molar mass lower than 50,000 and 150,000 g.mol−1 in the cases of polystyrene and polybutadiene, respectively (Polym. Int., 1991, 24, 197). Dilithium initiators based on different derivatives of double diphenylethylene-type molecules have been developed that are soluble in non-polar solvents. Although the addition reactions of these derivatives with sec-BuLi were found to be clean and rapid, the resulting dilithium initiators were insoluble forming fine suspensions, which would coagulate into hard particles after several hours (Macromolecules, 1994, 27, 2225; Macromolecules, 1994, 27, 1680; Macromolecules, 1994, 27, 2219). Finally, more complicated precursors such as α,ω-bis(phenylvinylidenyl)alkanes, 1,2-bis(isopropenyl-4-phenyl)ethane or (1,1,4,4-tetraphenyl)butane were developed and were found to be effective for the polymerization of dienes, but are difficult to synthesise (Polymer, 1981, 22, 1724; Polymer, 1982, 23, 73; Polymer, 1987, 28, 2093; Makromol. Chem., 1983, 184, 1983).
Whenever high contents of 1,4-polybutadiene or 1,4-cis-polyisoprene units are desired, the anionic polymerization of dienes has to be conducted in non polar solvent media, and with lithium as the counter-ion. sec-Butyllithium has proved to be an excellent monofunctional initiator. However, until now, no bifunctional initiator exhibiting carbon-lithium bonds, and yet soluble in non polar solvent media without additives has proved really satisfactory for the synthesis of polymers such as SBS or SIS thermoplastic elastomers and α,ω-difunctional polydienes with high 1,4-polydiene microstructure. Therefore, it is of great interest and importance to synthesise new bifunctional organolithium initiator soluble in non polar solvents and its use as an initiator for the synthesis of telechelic polydienes and polystyrenes and SBS or SIS triblock copolymers by anionic route with a high content of 1,4-units in polydienes without additives in non polar solvent.